The field of this invention is that of defogging mirror units and the invention relates more particularly to a defogging automobile mirror using an electrical resistance heater for defogging the mirror.
Mirror units intended for use on the exterior of automobiles have been provided with self-regulating electrical resistance heaters of materials of positive temperature coefficient of resistivity. The heaters are electrically energized when automobile operation is started for heating the glass-reflecting members of the mirror units so that the reflecting members are defogged. In a particularly advantageous defogging mirror unit of this type as shown in U.S. Pat. No. 4,237,366 commonly assigned to the assignee of the present invention, a self-regulating electrical resistance heater is mounted on a thermally and electrically conducting metal plate in a recess formed in the plate and a glass reflecting member having a reflecting surface formed thereon is mounted on the plate over the recess for enclosing the heater in the recess. In that arrangement, the heater is economically enclosed in the mirror unit to be conveniently connected in an electrical circuit for defogging the mirror.
However, it is found that when such electrical resistance heaters of positive temperature coefficient of resistance are energized, they initially provide a very substantial heat output. Then, they rapidly self heat and increase in resistance so that the heater current and heat output from the heater are reduced until the heater stabilizes at a self-regulated temperature at which the amount of heat generated by the heater is balanced by the amount of heat being dissipated from the heater. When such heaters are used for heating glass-reflecting members in a defogging mirror unit or the like, it is found they tend not to provide uniform heating to all portions of the mirror unit. Accordingly, there is some tendency to provide less than complete mirror defogging particularly under certain ambient temperature conditions and when the heater has been operating for a time in its stabilized, self-regulated, low heat output heating mode. On the other hand, if the heater is provided with a relatively higher operating temperature in attempting to overcome the incomplete mirror defogging, there is some risk of injury to the glass reflecting member due to the development of hot spots in the member.
It is an object of this invention to provide a novel and improved heated mirror unit; to provide a defogging mirror unit which is adapted to provide substantially uniform defogging of all portions of the unit in the various weather conditions likely to be encountered during operation of an automobile; to provide such mirror unit which is adapted to be operated under such varying weather conditions without risk of injury to the mirror unit; and to provide such an improved mirror unit which is of rugged and economical construction.